be concrete about abstractions - 國立臺灣大學htlin/course/oop09spring/...static revisited and...

Be Concrete about Abstractions

Hsuan-Tien Lin

Deptartment of CSIE, NTU

OOP Class, March 24, 2009

H.-T. Lin (NTU CSIE) Be Concrete about Abstractions OOP(even) 03/24/2009 0 / 55

Object/Class Lifecycle

Object Lifecycle (1/1)

1 c lass Record {2 i n t score ;3 Record ( i n t i n i t _ s c o r e ) { score = i n i t _ s c o r e ; }4 pro tec ted vo id f i n a l i z e ( ) throws Throwable { }5 }6 p u b l i c c lass RecordDemo {7 p u b l i c s t a t i c vo id main ( S t r i n g [ ] arg ) {8 Record r ; / / re ference declared9 Record r2 ; / / re ference declared

10 r = new Record ( 6 0 ) ; / / memory a l l o c a t e d (RHS)11 / / and cons t ruc to r c a l l e d12 / / re ference assigned (LHS)13 r2 = r ; / / re ference copied14 r . score = 3; / / ob jec t content accessed15 r . show_score ( ) ; / / ob jec t ac t i on performed16 r2 = n u l l ; r = n u l l ; / / memory s l o t i s o l a t e d17 / / . . . .18 / / f i n a l i z e r c a l l e d19 / / or JVM terminated20 }21 }

Object Lifecycle: Key Point

we control birth, life, death, funeral design, but not theexact funeral time (nor whether it would happen)

Class Lifecycle (1/1)

1 c lass Record {2 s t a t i c { / / c lass i n i t i a l i z e r3 count = 0 ; / / a c t u a l l y , done by d e f a u l t4 }5 s t a t i c i n t count ;6 i n t score ;7 Record ( i n t i n i t _ s c o r e ) { score = i n i t _ s c o r e ; count ++; }8 pro tec ted vo id f i n a l i z e ( ) throws Throwable { }9 / / c l a s s F i n a l i z e ? has been removed

10 }11 / / the c lass RecordDemo should be loaded a f t e r ’ java RecordDemo ’12 p u b l i c c lass RecordDemo {13 p u b l i c s t a t i c vo id main ( S t r i n g [ ] arg ) {14 / / the c lass Record should be loaded ( i n i t i a l i z e d ) before15 / / the f o l l o w i n g l i n e s16 Record r , r2 ;17 r = new Record ( 6 0 ) ;18 r2 = r ;19 / / . . . .20 / / by de fau l t , no unload ( and hence count would remain )21 / / i f unloaded , no guarantees22 }23 }

Class Lifecycle: Key Point

automatic and dynamic loading/linking in JVM:almost no need to care about load/unload

Methods: the Basic

Method (1/2, Callee’s View)

1 c lass Record {2 i n t score ;3 vo id add_to ( i n t i nc ) { score += inc ; }4 double get_adjusted ( ) { r e t u r n ( double ) ( score + 30 ) ; }5 }

method: what I (the object) doparameter: what I get from the callerreturn value: what I want to tell the caller

Methods: the Basic

Method (2/2, Caller’s View)

1 c lass Record {2 i n t score ;3 vo id add_to ( i n t i nc ) { score += inc ; }4 double get_adjusted ( ) { r e t u r n ( double ) ( score + 30 ) ; }5 }6 p u b l i c c lass RecordDemo {7 p u b l i c s t a t i c vo id main ( S t r i n g [ ] arg ) {8 Record r = new Record ( ) ;9 r . addto ( 5 0 ) ;

10 System . out . p r i n t l n ( r . get_ad justed ( ) ) ;11 }12 }

method: what I (caller) want the object to doargument: what I tell the calleereturn value: what I want to hear from the callee

Methods: the Basic

Method: Key Point

method: an abstraction of action, where informa-tion is passed through argument/parameter and re-turn values

Methods: Value Passing

Return Values (1/1)

1 c lass Record {2 S t r i n g name ;3 i n t score ;4 vo id add_to ( i n t i nc ) { score += inc ; }5 double get_adjusted ( ) { r e t u r n ( score + 30 ) ; }6 S t r i n g get_name ( ) { r e t u r n name ; }7 }8 p u b l i c c lass RecordDemo {9 p u b l i c s t a t i c vo id main ( S t r i n g [ ] arg ) {

10 Record r = new Record ( ) ;11 r . name = " HTLin " ;12 System . out . p r i n t l n ( r . get_name ( ) == r . name ) ;13 }14 }

void: no returnprimitive type: its valuereference type: its value (a.k.a. reference, not content)

Return Values: Key Point

Java: return by primitive/reference values

Primitive Argument/Parameter (1/1)

1 c lass Tool {2 vo id swap ( i n t f i r s t , i n t second ) {3 i n t tmp = f i r s t ;4 f i r s t = second ;5 second = tmp ;6 System . out . p r i n t l n ( f i r s t ) ;7 System . out . p r i n t l n ( second ) ;8 }9 }

10 p u b l i c c lass Demo{11 p u b l i c s t a t i c vo id main ( S t r i n g [ ] arg ) {12 Tool t = new Tool ( ) ;13 i n t i = 3 ; i n t j = 5 ;14 t . swap ( i , j ) ;15 System . out . p r i n t l n ( i ) ;16 System . out . p r i n t l n ( j ) ;17 }18 }

first, second: swappedi, j: didn’t

Primitive Argument/Parameter: Key Point

argument⇒ parameter: by value copying–change in parameter does not change argument

Reference Argument/Parameter (1/3)

1 c lass Tool {2 bool t r i c k y ( S t r i n g s1 , S t r i n g s2 ) {3 s2 = s2 + " " ;4 r e t u r n ( s1 == s2 ) ;5 }6 }7 p u b l i c c lass Demo{8 p u b l i c s t a t i c vo id main ( S t r i n g [ ] arg ) {9 Tool t = new Tool ( ) ;

10 S t r i n g sa = " HTLin " ;11 S t r i n g sb = sa ;12 System . out . p r i n t l n ( t . t r i c k y ( sa , sb ) ) ;13 System . out . p r i n t l n ( sa == sb ) ;14 System . out . p r i n t l n ( t . t r i c k y ( sa + " " , sb ) ) ;15 }16 }

reference parameter passing: again, value copyingsa, sb copied to s1, s2s2 (reference) changed, sb didn’t

1 c lass myInt { i n t va l ; myInt ( i n t v ) { va l = v ; } }2 c lass Tool {3 vo id swap ( myInt f i r s t , myInt second ) {4 i n t tmp = f i r s t . va l ;5 f i r s t . va l = second . va l ;6 second . va l = tmp ;7 System . out . p r i n t l n ( f i r s t . va l ) ;8 System . out . p r i n t l n ( second . va l ) ;9 }

10 }11 p u b l i c c lass Demo{12 p u b l i c s t a t i c vo id main ( S t r i n g [ ] arg ) {13 Tool t = new Tool ( ) ;14 myInt i = new myInt ( 3 ) ;15 myInt j = new myInt ( 5 ) ;16 t . swap ( i , j ) ;17 System . out . p r i n t l n ( i . va l ) ;18 System . out . p r i n t l n ( j . va l ) ;19 }20 }

swapped as requestedH.-T. Lin (NTU CSIE) Be Concrete about Abstractions OOP(even) 03/24/2009 13 / 55

1 c lass myInt { i n t va l ; myInt ( i n t v ) { va l = v ; } }2 c lass Tool {3 vo id swap ( myInt f i r s t , myInt second ) {4 myInt tmp = f i r s t ;5 f i r s t = second ;6 second = tmp ;7 System . out . p r i n t l n ( f i r s t . va l ) ;8 System . out . p r i n t l n ( second . va l ) ;9 }

10 }11 p u b l i c c lass Demo{12 p u b l i c s t a t i c vo id main ( S t r i n g [ ] arg ) {13 Tool t = new Tool ( ) ;14 myInt i = new myInt ( 3 ) ;15 myInt j = new myInt ( 5 ) ;16 t . swap ( i , j ) ;17 System . out . p r i n t l n ( i . va l ) ;18 System . out . p r i n t l n ( j . va l ) ;19 }20 }

what happens?H.-T. Lin (NTU CSIE) Be Concrete about Abstractions OOP(even) 03/24/2009 14 / 55

Reference Argument/Parameter: Key Point

argument⇒ parameter: by reference copying–value change: does not change argument–content change: change the “object”

Array Argument/Parameter (1/1)

1 c lass Tool {2 vo id swap ( i n t [ ] both ) {3 i n t tmp = both [ 0 ] ;4 both [ 0 ] = both [ 1 ] ;5 both [ 1 ] = tmp ;6 }7 }8 p u b l i c c lass Demo{9 p u b l i c s t a t i c vo id main ( S t r i n g [ ] arg ) {

10 Tool t = new Tool ( ) ;11 i n t [ ] a r r = new i n t [ 2 ] ;12 a r r [ 0 ] = 3 ; a r r [ 1 ] = 5 ;13 t . swap ( a r r ) ;14 System . out . p r i n t l n ( a r r [ 0 ] ) ;15 System . out . p r i n t l n ( a r r [ 1 ] ) ;16 }17 }

array is just special reference, same calling mechanism

Array Argument/Parameter: Key Point

argument⇒ parameter: by reference value copying–same as reference argument/parameter

Static Revisited and Beyond

Static Variables Revisited (1/1)

1 c lass Record {2 s t a t i c i n t t o t a l _ r e c = 0;3 i n t i d ;4 p u b l i c Record ( ) { i d = t o t a l _ r e c ++; }5 }6 p u b l i c c lass RecordDemo {7 p u b l i c s t a t i c vo id main ( S t r i n g [ ] arg ) {8 Record r1 = new Record ( ) ;9 Record r2 = n u l l ;

10 Record r3 = new Record ( ) ;11 System . out . p r i n t l n ( r1 . t o t a l _ r e c ) ;12 System . out . p r i n t l n ( r2 . t o t a l _ r e c ) ;13 System . out . p r i n t l n ( Record . t o t a l _ r e c ) ;14 System . out . p r i n t l n ( r1 . i d ) ;15 System . out . p r i n t l n ( r2 . i d ) ;16 System . out . p r i n t l n ( Record . i d ) ;17 }18 }

r2.total_rec⇒ Record.total_rec in compile time

Static Variables Revisited: Key Point

static variable:of the class (shared), not of an object

Static Methods (1/2)

1 c lass myMath {2 double mean( double a , double b ) {3 r e t u r n ( a + b ) ∗ 0 . 5 ;4 }5 }6 p u b l i c c lass MathDemo{7 p u b l i c s t a t i c vo id main ( S t r i n g [ ] arg ) {8 double i = 3 . 5 ;9 double j = 2 . 4 ;

10 myMath m = new MyMath ( ) ;11 System . out . p r i n t l n (m. mean( i , j ) ) ;12 }13 }

new a myMath object just for computing mean–lazy people don’t want to do so

Static Methods (2/2)

1 c lass myMath {2 s t a t i c double mean( double a , double b ) {3 r e t u r n ( a + b ) ∗ 0 . 5 ;4 }5 }6 p u b l i c c lass MathDemo{7 p u b l i c s t a t i c vo id main ( S t r i n g [ ] arg ) {8 double i = 3 . 5 ;9 double j = 2 . 4 ;

10 System . out . p r i n t l n (myMath . mean( i , j ) ) ;11 System . out . p r i n t l n ( ( new myMath ( ) ) . mean( i , j ) ) ;12 }13 }

make the method a static (class) one–no need to new an objectsimilar to static variable usage

Static Methods: Key Point

static method:associated with the class,no need to create an object

Use of Static Methods (1/3)

1 p u b l i c c lass UtilDemo {2 p u b l i c s t a t i c vo id main ( S t r i n g [ ] arg ) {3 System . out . p r i n t l n ( Math . PI ) ;4 System . out . p r i n t l n ( Math . s q r t ( 2 . 0 ) ) ;5 System . out . p r i n t l n ( Math .max( 3 . 0 , 5 . 0 ) ) ;6 System . out . p r i n t l n ( I n tege r . t o B i n a r y S t r i n g ( 1 5 ) ) ;7 }8 }

commonly used as utility functions(so don’t need to create object)

1 c lass Record {2 s t a t i c i n t t o t a l _ r e c = 0;3 Record ( ) { t o t a l _ r e c ++; }4 s t a t i c vo id show_tota l_rec ( ) {5 System . out . p r i n t l n ( t o t a l _ r e c ) ;6 }7 }8 p u b l i c c lass RecordDemo {9 p u b l i c s t a t i c vo id main ( S t r i n g [ ] arg ) {

10 Record r1 = new Record ( ) ;11 Record . show_tota l_rec ( ) ;12 }13 }

class related actions rather than object related actions

1 p u b l i c c lass MainDemo{2 s t a t i c vo id p r i n t S t r ( S t r i n g s ) {3 System . out . p r i n t ( s ) ;4 }5 p u b l i c s t a t i c vo id main ( S t r i n g [ ] arg ) {6 p r i n t S t r ( " I am main ; I am also s t a t i c " ) ;7 }8 }

main is static (called by classname during ’java className’)as tools for other static methods

Use of Static Methods: Key Point

static method:compile time determinedper classsometimes useful

Instance versus Class

Instance versus Class (1/2)

1 c lass Record {2 s t a t i c i n t count ;3 i n t i d ;4 Record ( ) { i d = count ++; }5 s t a t i c vo id show_int ( i n t num ) {6 System . out . p r i n t l n (num ) ;7 }8 s t a t i c vo id show_count ( ) {9 show_int ( count ) ;

10 }11 vo id show_id ( ) {12 show_int ( i d ) ;13 }14 vo id show_id_and_count ( ) {15 show_id ( ) ;16 show_count ( ) ;17 }18 }

instance methods: can access all members/methodsstatic methods: can access static members/methodsH.-T. Lin (NTU CSIE) Be Concrete about Abstractions OOP(even) 03/24/2009 27 / 55

Instance versus Class (2/2)

1 / / t h i s : means my2 c lass Record {3 s t a t i c i n t count ; i n t i d ;4 Record ( ) { t h i s . i d = Record . count ++; }5 s t a t i c vo id show_int ( i n t num ) {6 java . lang . System . out . p r i n t l n (num ) ;7 }8 s t a t i c vo id show_count ( ) {9 Record . show_int ( Record . count ) ;

10 }11 vo id show_id ( ) {12 Record . show_int ( t h i s . i d ) ;13 }14 vo id show_id_and_count ( ) {15 t h i s . show_id ( ) ; Record . show_count ( ) ;16 }17 }

what compiler sees: a fully qualified intentionthis: the “object” reference (known in run-time)Record: the “class” name (known in compile-time)H.-T. Lin (NTU CSIE) Be Concrete about Abstractions OOP(even) 03/24/2009 28 / 55

Instance versus Class: Key Point

null/non-null instance access classmethod/variable: as if YESclass access instance method/variable method:NOinstance method access class method/variable:YESclass method access instance method/variable:NO

this (1/3)

1 c lass Record {2 s t a t i c i n t count ; i n t i d ;3 Record ( ) { t h i s . i d = Record . count ++; }4 s t a t i c vo id show_int ( i n t num ) {5 java . lang . System . out . p r i n t l n (num ) ;6 }7 s t a t i c vo id show_id ( Record r ) {8 Record . show_int ( r . i d ) ;9 }

10 vo id or ig_show_id ( ) {11 Record . show_int ( t h i s . i d ) ;12 }13 }14 p u b l i c c lass RecordDemo {15 p u b l i c s t a t i c vo id main ( S t r i n g [ ] arg ) {16 Record r = new Record ( ) ;17 r . or ig_show_id ( ) ; Record . show_id ( r ) ;18 }19 }

a static implementation of show_id“almost” what the compiler/JVM doesH.-T. Lin (NTU CSIE) Be Concrete about Abstractions OOP(even) 03/24/2009 30 / 55

this (2/3)

1 c lass Record {2 s t a t i c i n t count ; i n t i d ;3 Record ( ) { t h i s . i d = Record . count ++; }4 s t a t i c vo id show_int ( i n t num ) {5 java . lang . System . out . p r i n t l n (num ) ;6 }7 s t a t i c vo id show_id ( Record THIS ) {8 Record . show_int ( THIS . i d ) ;9 }

10 vo id or ig_show_id ( ) {11 Record . show_int ( t h i s . i d ) ;12 }13 }14 p u b l i c c lass RecordDemo {15 p u b l i c s t a t i c vo id main ( S t r i n g [ ] arg ) {16 Record r = new Record ( ) ;17 r . or ig_show_id ( ) ; Record . show_id ( r ) ;18 }19 }

implicitly, instance method can access an additionalreference-type parameter this passed from the callerH.-T. Lin (NTU CSIE) Be Concrete about Abstractions OOP(even) 03/24/2009 31 / 55

this (3/3)

1 c lass Record {2 i n t score ;3 s t a t i c vo id show_score ;4 vo id se t_ to ( i n t score ) { t h i s . score = score ; }5 }

this: the reference to specifically say “my” variable/method

this: Key Point

this: the reference used to specifically say “mine”—implicit in every instance method

Recursive Calls

Recursive Calls (1/5)

1 c lass D i r { F i l e [ ] f i l e s ; D i r [ ] d i r s ; }2 c lass F i l e { S t r i n g name ; }3 p u b l i c c lass DirDemo {4 s t a t i c vo id l i s t A l l ( D i r cu r ren t ) {5 i n t i , j ;6 f o r ( i =0; i < f i l e s . leng th ; i ++)7 System . out . p r i n t l n ( f i l e s [ i ] . name ) ;8 f o r ( j =0; j < d i r s . leng th ; j ++)9 l i s t A l l ( d i r s [ j ] ) ;

10 }11 p u b l i c s t a t i c vo id main ( S t r i n g [ ] arg ) {12 / / . . .13 Di r d = new Di r ( ) ;14 l i s t A l l ( d ) ;15 }16 }

recursive: when a method calls itself

Recursive Calls

1 p u b l i c c lass FibDemo {2 s t a t i c i n t f i b ( i n t n ) {3 i n t res ;4 System . out . p r i n t l n ( " f i b ( " + n + " ) c a l l e d " ) ;5 i f ( n <= 1)6 res = 1;7 else8 res = f i b ( n−1) + f i b ( n−2);9 System . out . p r i n t l n ( " f i b ( " + n + " ) r e t u r n i n g " ) ;

10 r e t u r n res ;11 }12 p u b l i c s t a t i c vo id main ( S t r i n g [ ] arg ) {13 System . out . p r i n t l n ( f i b ( 5 ) ) ;14 }15 }

method call: do last task first (recursive or non-recursive)method call stack: implement “last task first” (last-in-first-out)

Recursive Calls

1 p u b l i c c lass FibDemo {2 s t a t i c i n t f i b ( i n t n ) {3 i n t res ;4 System . out . p r i n t l n ( " f i b ( " + n + " ) c a l l e d " ) ;5 i f ( n <= 1)6 res = 1;7 else {8 i n t m = n−1; res = f i b (m) ; res = res + f i b ( n−2);9 }

10 System . out . p r i n t l n ( " f i b ( " + n + " ) r e t u r n i n g " ) ;11 r e t u r n res ;12 }13 p u b l i c s t a t i c vo id main ( S t r i n g [ ] arg ) { i n t a = f i b ( 5 ) ; }14 }

what needs to be stored in each “frame”?my local version of nmy local version of resany temporary value generated by compiler (e.g. like m)(my return value and where I am returning to)

Recursive Calls

1 p u b l i c c lass Fib {2 i n t N;3 Fib ( i n t N) { t h i s .N = N; }4 i n t get ( ) {5 i f (N <= 1)6 r e t u r n 1 ;7 else8 r e t u r n (new Fib (N−1)) . get ( ) + (new Fib (N−2)) . get ( ) ;9 }

10 p u b l i c s t a t i c vo id main ( S t r i n g [ ] arg ) {11 Fib f = new Fib ( 2 0 ) ;12 System . out . p r i n t l n ( " res = " + f . get ( 5 ) ) ;13 System . out . p r i n t l n ( " res = " + f . get (10) ) ;14 }15 }

one advanced use: one class, two behaviorsutility behavior: static mainobject behavior: with a state

is get recursive? seems No (different objects) but actually Yes(same method with different “this”)H.-T. Lin (NTU CSIE) Be Concrete about Abstractions OOP(even) 03/24/2009 37 / 55

Recursive Calls

1 p u b l i c c lass Fib {2 i n t MAXN; i n t [ ] computed ;3 Fib ( i n t MAXN) {4 t h i s .MAXN = MAXN; computed = new i n t [MAXN+1]5 }6 i n t get ( i n t n ) {7 i f ( computed [ n ] == 0 ) {8 i f ( n <= 1) computed [ n ] = 1 ;9 else computed [ n ] = get ( n−1) + get ( n−2);

10 }11 r e t u r n computed [ n ] ;12 }13 p u b l i c s t a t i c vo id main ( S t r i n g [ ] arg ) {14 Fib f = new Fib ( 2 0 ) ;15 System . out . p r i n t l n ( " res = " + f . get ( 5 ) ) ;16 System . out . p r i n t l n ( " res = " + f . get (10) ) ;17 }18 }

recursive, but no repeated computationno need to create so many objectsthink: any better implementations?H.-T. Lin (NTU CSIE) Be Concrete about Abstractions OOP(even) 03/24/2009 38 / 55

Recursive Calls

Recursive Calls: Key Point

method call: not just “goto”—comes with a frame of status passing, storing, ma-nipulating, and returning

Local Variables

Local Variables (1/7)

1 p u b l i c c lass FibDemo {2 i n t f i b ( i n t n ) {3 i n t res ;4 S t r i n g s = " f i b ( " + n + " ) c a l l e d " ;5 System . out . p r i n t l n ( s ) ;6 i f ( n <= 1)7 res = 1;8 else {9 res = f i b ( n−1);

10 res = res + f i b ( n−2);11 }12 r e t u r n res ;13 }14 p u b l i c s t a t i c vo id main ( S t r i n g [ ] arg ) {15 FibDemo f = new FibDemo ( ) ;16 System . out . p r i n t l n ( " res = " + f . f i b ( 5 ) ) ;17 }18 }

local primitive n: allocated, and assigned by argument⇒parameter

Local Variables

local referece this: allocated, and assigned by argument (f)⇒parameter (this)

Local Variables

local primitive res: allocated, not initialized assigned by ourselves

Local Variables

local reference s: reference allocated, not initialized, point to avalid object by ourselves

Local Variables

some other local variables generated by compiler: allocated, notinitialized, used internally

Local Variables

1 p u b l i c c lass FibDemo {2 i n t f i b ( i n t n ) {3 i n t res ;4 S t r i n g s = " f i b ( " + n + " ) c a l l e d " ;5 System . out . p r i n t l n ( s ) ;6 i f ( n <= 1) res = 1;7 else res = f i b ( n−1) + f i b ( n−2);8 r e t u r n res ;9 }

10 p u b l i c s t a t i c vo id main ( S t r i n g [ ] arg ) {11 FibDemo f = new FibDemo ( ) ;12 System . out . p r i n t l n ( " res = " + f . f i b ( 5 ) ) ;13 }14 }

when call returns: the result is “copied” to the previous framesomewherelocal primitive: simply discardedlocal reference: discarded (and then the “object” GC’ed some timeif no longer used)H.-T. Lin (NTU CSIE) Be Concrete about Abstractions OOP(even) 03/24/2009 45 / 55

Local Variables

Local Variables (7/7, courtesy of Prof. Chuen-Liang Chen)

localvariable

instancevariable

class (static)variable

b l t method i t lbelong to methodinvocation instance class

declaration within method within class within classdifi i O O Smodifier static NO NO YES

allocation (when) methodinvocation

instancecreation

classloadingg

allocation (where) stack memory heap memory heap memoryinitial to 0 NO YES YES

de-allocation methodreturn

automaticgarbage

collectionNO

usage range direct access rangefrom

declaration to whole class whole class

Chuen-Liang Chen, NTU CS&IE / 5OOP

declaration to end of block

whole class whole class

Local Variables

Local Variables: Key Point

local variables: the “status” of the current frame—by spec not necessarily initialized

be concrete about abstractions - 國立臺灣大學htlin/course/oop09spring/...static revisited and...

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